Organic light - emitting diodes and methods for assembly and enhanced charge injection

ABSTRACT

New organic light-emitting diodes and related electroluminescent devices and methods for fabrication, using siloxane self-assembly techniques.

[0001] This application is a continuation-in-part of and claims prioritybenefit from application Ser. No. 09/187,891, filed on Nov. 6, 1998,which is a continuation-in-part of application Ser. No. 08/673,600,filed on Jun. 25, 1996, and issued as U.S. Pat. No. 5,834,100, each ofwhich are incorporated herein by reference in their entirety.

[0002] The United States Government has certain rights to this inventionpursuant to Grant Nos. N0014-95-1-1319 and DMR-00769097 from the Officeof Naval Research and National Science Foundation, respectively, toNorthwestern University.

BACKGROUND OF THE INVENTION

[0003] This invention relates generally to organic electroluminescentdevices with organic films between anodic and cathodic electrodes, andmore particularly to such devices and methods for their assembly usingthe condensation of various silicon moieties.

[0004] Organic electroluminescent devices have been known, in variousdegrees of sophistication, since the early 1970's. Throughout theirdevelopment and consistent with their function and mode of operation,they can be described generally by way of their physical construction.Such devices are characterized generally by two electrodes which areseparated by a series of layered organic films that emit light when anelectric potential is applied across the two electrodes. A typicaldevice can consist, in sequence, of an anode, an organic hole injectionlayer, an organic hole transport layer, an organic electron transportlayer, and a cathode. Holes are generated at a transparent electrode,such as one constructed of indium-tin-oxide, and transported through ahole-injecting or hole-transporting layer to an interface with anelectron-transporting or electron-injecting layer which transportselectrons from a metal electrode. An emissive layer can also beincorporated at the interface between the hole-transporting layer andthe electron-transporting layer to improve emission efficiency and tomodify the color of the emitted light.

[0005] Significant progress has been made in the design and constructionof polymer- and molecule-based electroluminescent devices, forlight-emitting diodes, displays and the like. Other structures have beenexplored and include the designated “DH” structure which does notinclude the hole injection layer, the i“SH-A” structure which does notinclude the hole injection layer or the electron transport layer, andthe “SH-B” structure which does not include the hole injection layer orthe hole transport layer. See, U.S. Pat. No. 5,457,357 and in particularcol. 1 thereof, which is incorporated herein by reference in itsentirety.

[0006] The search for an efficient, effective electroluminescent deviceand/or method for its production has been an ongoing concern. Severalapproaches have been used with certain success. However, the prior arthas associated with it a number of significant problems anddeficiencies. Most are related to the devices and the methods by whichthey are constructed, and result from the polymeric and/or molecularcomponents and assembly techniques used therewith.

[0007] The fabrication of polymer-based electroluminescent devicesemploys spin coating techniques to apply the layers used for the device.This approach is limited by the inherently poor control of the layerthickness in polymer spin coating, diffusion between the layers,pinholes in the layers, and inability to produce thin layers which leadsto poor light collection efficiency and the necessity of high D.C.driving voltages. The types of useful polymers, typicallypoly(phenylenevinylenes), are greatly limited and most areenvironmentally unstable over prolonged use periods.

[0008] The molecule-based approach uses vapor deposition techniques toput down thin films of volatile molecules. It offers the potential of awide choice of possible building blocks, for tailoring emissive andother characteristics, and reasonably precise layer thickness control.Impressive advances have recently been achieved in molecular buildingblocks—especially in electron transporters and emitters, layer structuredesign (three versus two layers), and light collection/transmissionstructures (microcavities).

[0009] Nevertheless, further advances must be made before these devicesare optimum. Component layers which are thinner than achievable byorganic vapor deposition techniques would allow lower DC drivingvoltages and better light transmission collection characteristics. Manyof the desirable component molecules are nonvolatile or poorly volatile,with the latter requiring expensive, high vacuum or MBE growthequipment. Such line-of-site growth techniques also have limitation interms of conformal coverage. Furthermore, many of the desirablemolecular components do not form smooth, pinhole-free, transparent filmsunder these conditions nor do they form epitaxial/quasiepitaxialmultilayers having abrupt interfaces. Finally, the mechanical stabilityof molecule-based films can be problematic, especially for large-areaapplications or on flexible backings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIGS. 1A and 1B show structural formulae for porphyrinic compoundswhich are illustrative examples of compounds of the type which can beused as hole injection components/agents in the preparation of themolecular conductive or hole injection layers and electroluminescentmedia of this invention. In FIG. 1, M is Cu, Zn, SiCl₂, or 2H; Q is N orC(X), where X is a substituted or unsubstituted alkyl or aryl group; andR is H, trichlorosilyl, trialkoxysilyl, or a moiety having 1 to 6 carbonatoms which can include trichlorosilyl or trialkoxysilyl groups,substituted on the C₁-C₄, C₈-C₁₁, C₁₅-C₁₈ and/or C₂₂-C₂₅ positions. InFIG. B, M is Cu, Zn, SiCl₂, or 2H; Q is N or C(X), where X is asubstituted or unsubstituted alkyl or aryl group; and T₁/T₂ is H,trichlorosilyl, trialkoxysilyl, or a moiety having 1 to 6 carbon atomswhich can include trichlorosilyl or trialkoxysilyl groups.

[0011] FIGS. 2A-2C show structural formulae for arylamine compoundswhich are illustrative examples of compounds of the type which can beused as hole transport compounds/agents in the preparation of themolecular conductive or hole transport layers and electroluminescentmedia of this invention. In FIG. 2A, R₂, R₃ and/or R₄ can be H,trihalosilyl, trialkoxysilyl, dihalosilyl, dialkoxysilyl, or a moietyhaving 1 to 6 carbon atoms which can include dialkyldichlorosilyl,dialkyldialkoxysilyl, trichlorosilyl or trialkoxysilyl groupssubstituted anywhere on the aryl positions. In FIG. 2B, Q₁ and Q₂ can besubstituted or unsubstituted tertiary aryl amines, such as thosedescribed with FIG. 2A; and G is a linking group to include but notlimited to an alkyl, aryl, cylcohexyl or heteroatom group. In FIG. 2C,Ar is an arylene group; n is the number of arylene groups from 1-4; andR₅, R₆, R₇, and/or R₈ can be H, trihalosilyl, trialkoxysilyl,dihalosilyl, dialkoxysilyl or a moiety having 1 to 6 carbon atoms whichcan include dialkyldichlorosilyl, dialkyldialkoxysilyl, trichlorosilylor trialkoxysilyl groups substituted anywhere on the aryl positions.

[0012] FIGS. 3A-3C show structural formulae for aryl compounds which areillustrative of examples of compounds of the type which can be used asemissive compounds/agents in the preparation of the molecular conductivelayers and electroluminescent media of this invention. In FIG. 3A, R₉and R₁₀ can be H, trihalosilyl, trialkoxysilyl, dihalosilyl,dialkoxysilyl, or a moiety having 1 to 6 carbon atoms which can includedialkyldichlorosilyl, dialkyldialkoxysilyl, trichlorosilyl ortrialkoxysilyl groups substituted anywhere on the aryl positions. InFIG. 3B, M is Al or Ga; and R₁₁-R₁₄ can be H, trihalosilyl,trialkoxysilyl, dihalosilyl, dialkoxysilyl, or a moiety having 1 to 6carbon atoms which can include dialkyldichlorosilyl,dialkyldialkoxysilyl, trichlorosilyl or trialkoxysilyl groupssubstituted anywhere on the aryl positions. In FIG. 3C, Ar is arylene;and R₁₅-R₁₈ can be H, trihalosilyl, trialkoxysilyl, dihalosilyl,dialkoxysilyl, or a moiety having 1 to 6 carbon atoms which can includedialkyldichlorosilyl, dialkyldialkoxysilyl, trichlorosilyl ortrialkoxysilyl groups substituted anywhere on the aryl positions.

[0013] FIGS. 4A-4C show structural formulae for heterocyclic compoundswhich are illustrative examples of compounds of the type which can beused as electron transport components/agents in the preparation of themolecular conductive or electron transport layers and inelectroluminescent media of this invention. In FIGS. 4A-4C, X is O or S;and R₁₉-R₂₄ can be aryl groups substituted with the followingsubstituents anywhere on the aryl ring: trihalosilyl, trialkoxysilyl,dihalosilyl, dialkoxysilyl, or a moiety having 1 to 6 carbon atoms whichcan contain dialkyldichlorosilyl, dialkyldialkoxysilyl, trichlorosilylor trialkoxysilyl groups.

[0014]FIGS. 5A and 5B (ITO is indium-tin-oxide; HTL is hole transportlayer and ETL is electron transport layer) show, schematically and in astep-wise manner by way of illustrating the present invention, use ofthe components/agents of Examples 1-5 and FIGS. 1-4 in the self-assemblyand preparation of an organic light-emitting diode device. Inparticular, the molecular representation FIG. 5A illustrates thehydrolysis of an assembled silicon/silane component/agent to provide anSi—OH functionality reactive toward a silicon/silane moiety of anothercomponent, agent or conductive layer. The block and molecularrepresentations of FIG. 5B illustrate a completed assembly.

[0015]FIG. 6 shows an alternative synthetic sequence enroute to severalarylamine components/agents, also in accordance with the presentinvention.

[0016]FIG. 7 shows, schematically and by way of illustrating analternative embodiment of the present invention, use of thecomponents/agents of FIG. 6 in the preparation of another representativeelectroluminescent device.

[0017]FIG. 8 graphically correlates x-ray reflectivity measurements offilm thickness with the number of capping layers applied to a substrate.As calculated from the slope of the line (y=8.3184x), each layer isabout 7.84 Å in dimensional thickness.

[0018]FIG. 9 graphically shows cyclic voltametry measurements, using10⁻³M ferrocene in acetonitrile, taken after successive layer (c-e)deposition and as compared to a bare ITO electrode (a). Even one cappinglayer (b), in accordance with this invention, effectively blocks theelectrode surface. Complete blocking is observed after deposition ofthree or four layers. The sweep rate was 100 mV/sec, and the electrodearea was about 0.7 cm².

[0019] FIGS. 10A-C graphically illustrate various utilities and/orperformance characteristics (current density, quantum efficiency andforward light output, respectively, versus voltage) achievable throughuse of the present invention, as a function of the number of cappinglayers on an electrode surface. 0 layers, bare ITO (<>), 1 layer, 8 Å(□), 2 layers, 17 Å (), 3 layers, 25 Å (▴) and 4 layers, 33 Å (∇).Reference is made to example 10.

[0020]FIG. 11A shows molecular structures of hole adhesion/injectionmolecular components: TAA (shown after crosslinking), TPD-Si₂ (shownafter crosslinking), and prior art copper phthalocyanine, Cu(Pc). FIG.11B illustrates one possible scheme for the synthesis of a preferredTPD-Si₂ adhesion/injection interlayer molecular precursor. Reference isalso made to the procedures described in Example 2.

[0021] FIGS. 12A-D provide optical microscopic images of vapor-depositedTPD film (100 nm) morphology after annealing at 80° C. for 1.0 h on ITOsubstrates coated with a cured 40 nm thick TPD-Si₂ film (12A) and onbare ITO (12B); polarized optical image of TPD film (100 nm) morphologybefore (12C) and after (12D) annealing the bilayer structure:ITO/CuPc(10 nm)/TPD (100 nm) at 80° C. for 0.50 h.

[0022] FIGS. 13A-C show, in turn, (A) light output, (B) external quantumefficiency, and (C) current-voltage characteristics as a function ofoperating voltage for OLED devices having the structure:ITO/(adhesion/injection/molecular component interlayer)/TPD holetransport layer (50 nm)/Alq (60 nm)/Al, where the injection/adhesioncomponent interlayer is prior art Cu(Pc) (10 nm), TAA (15 nm), andTPD-Si₂ (40 nm).

[0023]FIG. 14 compares injection characteristics of hole-only deviceshaving the structure ITO/molecular component interlayer/TPD (250nm)/Au(5 nm)/Al (180 nm) for various anode functionalization layers.

[0024]FIG. 15 graphically illustrates by comparison the effect ofthermal stressing (90° C. under vacuum) on device characteristics ofITO/(injection/adhesion interlayer)/TPD(50 nm)/Alq(60 nm)/Al (100 nm)devices, where the molecular component interlayer is TPD-Si₂ (40 nm),TAA (15 nm), and prior art CuPc (10 nm).

SUMMARY OF THE INVENTION

[0025] In light of the foregoing, it is an object of the presentinvention to provide electroluminescent articles and/or devices andmethod(s) for their production and/or assembly, thereby overcomingvarious deficiencies and shortcomings of the prior art, including thoseoutlined above. It will be understood by those skilled in the art thatone or more aspects of this invention can meet certain objectives, whileone or more other aspects can meet certain other objectives. Eachobjective may not apply equally, in all its respects, to every aspect ofthis invention. As such, the following objects can be viewed thealternative with respect to any one aspect of this invention.

[0026] It is an object of the present invention to provide control overthe thickness dimension of a luminescent medium and/or the conductivelayers of such a medium, to control the wavelength of light emitted fromany electroluminescent device and enhance the efficiency of suchemission.

[0027] It can be another object of the present invention to providemolecular components for the construction and/or modification of anelectroluminescent medium and/or the conductive layers thereof, whichwill allow lower driving and/or turn-on voltages than are availablethrough use of conventional materials.

[0028] It can also be an object of the present invention to providecomponent molecules which can be used effectively in liquid mediawithout resort to high vacuum or MBE growth equipment.

[0029] It can also be an object of the present invention to provideconformal conductive layers and the molecular components thereof whichallows for the smooth, uniform deposition on an electrode, substratesurface and/or previously-deposited layers.

[0030] It can also be an object of this invention to provide anelectroluminescent medium having a hybrid structure and where one ormore of the layers is applied by a spin-coat or vapor depositiontechnique to one or more self-assembled conductive layers.

[0031] Other objects, features and advantages of the present inventionwill be apparent from this summary of the invention and its descriptionsof various preferred embodiments, and will be readily apparent to thoseskilled in the art having knowledge of various electroluminescentdevices and assembly/production techniques. Such objects, features,benefits and advantages will be apparent from the above as taken intoconjunction with the accompanying examples, data, figures and allreasonable inferences to be drawn therefrom, alone or with considerationof the references incorporated herein.

[0032] This invention describes, in part, a new route to the fabricationof light-emitting organic multilayer heterojunction devices, useful forboth large and small, multicolored display applications. As describedmore fully below, electron and hole transporting layers, as well as theemissive layer, as well as any other additional layers, are applied,developed and/or modified by molecular self-assembly techniques. Assuch, the invention can provide precise control over the thickness of aluminescent medium or the conductive layers which make up such a medium,as well as provide maximum light generation efficiency. Use of thepresent invention provides strong covalent bonds between the constituentmolecular components, such that the mechanical, thermal, chemical and/orphotochemical stability of such media and/or conductive layers, as canbe used with an electroluminescent device, are enhanced. The use of suchcomponents also promotes conformal surface coverage to prevent cracksand pinhole deformities.

[0033] More specifically, the siloxane self-assembly techniquesdescribed herein allow for the construction of molecule-basedelectroluminescent media and devices. As described more fully below,various molecular components can be utilized to control the thicknessdimension of the luminescent media and/or conductive layers. Nanometerdimensions can be obtained, with self-sealing, conformal coverage. Theresulting covalent, hydrophobic siloxane network imparts considerablemechanical strength, as well as enhancing the resistance of such mediaand/or devices to dielectric breakdown, moisture intrusion, and otherdegradative processes.

[0034] In part, the present invention is an electroluminescent articleor device which includes (1) an anode, (2) a plurality of molecularconductive layers where one of the layers is coupled to the anode withsilicon-oxygen bonds and each of the layers is coupled one to anotherwith silicon-oxygen bonds, and (3) a cathode in the electrical contactwith the conductive layers. More generally and within the scope of thisinvention, an anode is separated from a cathode by an organicluminescent medium. The anode and the cathode are connected to anexternal power source by conductors. The power source can be acontinuous direct, alternating or an intermittent current voltagesource. A convenient conventional power source, including any desiredswitching circuitry, which is capable of positively biasing the anodewith respect to the cathode, can be employed. Either the anode orcathode can be at ground potential.

[0035] The conductive layers can include but are limited to a holetransport layer, a hole injection layer, an electron transport layer andan emissive layer. Under forward biasing conditions, the anode is at ahigher potential than the cathode, and the anode injects holes (positivecharge carriers) into the conductive layers and/or luminescent mediumwhile the cathode injects electrons therein. The portion of thelayers/medium adjacent to the anode forms a hole injecting and/ortransporting zone while the portion of the layers/medium adjacent to thecathode forms an electron injecting and/or transporting zone. Theinjected holes and electrons each migrate toward the oppositely chargedelectrode, resulting in hole-electron interaction within the organicluminescent medium of conductive layers. A migrating electron drops fromits conduction potential to a valence band in filling a hole to releaseenergy as light. In such a manner, the organic luminescent layers/mediumbetween the electrodes performs as a luminescent zone receiving mobilecharge carriers from each electrode. Depending upon the construction ofthe article/device, the released light can be emitted from theluminescent conductive layers/medium through one or more of edgesseparating the electrodes, through the anode, through the cathode, orthrough any combination thereof. See, U.S. Pat. No. 5,409,783 and, inparticular cols. 4-6 and FIG. 1 thereof, which is incorporated herein byreference in its entirety. As would be understood by those skilled inthe art, reverse biasing of the electrodes will reverse the direction ofmobile charge migration, interrupt charge injection, and terminate lightemission. Consistent with the prior art, the present inventioncontemplates a forward biasing DC power source and reliance on externalcurrent interruption or modulation to regulate light emission.

[0036] As demonstrated and explained below, it is possible to maintain acurrent density compatible with efficient light emission while employinga relatively low voltage across the electrodes by limiting the totalthickness of the organic luminescent medium to nanometer dimensions. Atthe molecular dimensions possible through use of this invention, anapplied voltage of less than about 10 volts is sufficient for efficientlight emission. As discussed more thoroughly herein, the thickness ofthe organic luminescent conductive layers/medium can be designed tocontrol and/or determine the wavelength of emitted light, as well asreduce the applied voltage and/or increase in the field potential.

[0037] Given the nanometer dimensions of the organic luminescentlayers/medium, light is usually emitted through one of the twoelectrodes. The electrode can be formed as a translucent or transparentcoating, either on the organic layer/medium or on a separate translucentor transparent support. The layer/medium thickness is constructed tobalance light transmission (or extinction) and electrical conductance(or resistance). Other considerations relating to the design,construction and/or structure of such articles or devices are asprovided in the above referenced U.S. Pat. No. 5,409,783, suchconsiderations as would be modified in accordance with the molecularconductive layers and assembly methods of the present invention.

[0038] In preferred embodiments, the conductive layers have molecularcomponents, and each molecular component has at least two siliconmoieties. In highly preferred embodiments, each silicon moiety is ahalogenated or alkoxylated silane and silicon-oxygen bonds areobtainable from the condensation of the silane moieties with hydroxyfunctionalities. In preferred embodiments, the present invention employsan anode with a substrate having a hydroxylated surface portion. Thesurface portion is transparent to near-IR and visible wavelengths oflight. In such highly preferred embodiments the hydroxylated surfaceportions include SiO₂, In₂.xSnO₂, Ge and Si, among other such materials.

[0039] In conjunction with anodes and the hydroxylated surface portionsthereof, the conductive layers include molecular components, and eachmolecular component has at least two silicon moieties. As discussedabove, in such embodiments, each silicon moiety is a halogenated oralkoxylated silane, and silicon-oxygen bonds are obtainable from thecondensation of the silane moieties with hydroxy functionalities whichcan be on a surface portion of an anode. Consistent with such preferredembodiments, a cathode is in electrical contact with the conductivelayers. In highly preferred embodiments, the cathode is vapor depositedon the conductive layers, and constructed of a material including Al,Mg, Ag, Au, In, Ca and alloys thereof.

[0040] In part, the present invention is a method of producing alight-emitting diode having enhanced stability and light generationefficiency. The method includes (1) providing an anode with ahydroxylated surface; (2) coupling the surface to a hole transport layerhaving a plurality of molecular components, with each component havingat least two silicon moieties reactive with the surface, with couplingof one of the silicon moieties to form silicon-oxygen bonds between thesurface and the hole transport layer; (3) coupling the hole transportlayer to an electron transport layer, the electron transport layerhaving a plurality of molecular components with each of the componentshaving at least two silicon moieties reactive with the hole transportlayer, with the coupling of one of the silicon moieties to formsilicon-oxygen bonds between the hole and electron transport layers; and(4) contacting the electron transport layer with a cathode material.

[0041] In preferred embodiments of this method, the hole transport layerincludes a hole injecting zone of molecular components and a holetransporting zone of molecular components. Likewise, in preferredembodiments, each silicon moiety is a halogenated or alkoxylated silanesuch that, with respect to this embodiment, coupling the hole transportlayer to the electron transport layer further includes hydrolyzing thehalogenated or alkoxylated silane. Likewise, with respect to ahalogenated or alkoxylated silane embodiment, contacting the electrontransport layer with the cathode further includes hydrolyzing thesilane.

[0042] In part, the present invention is a method of controlling thewavelength of light emitted from an electroluminescent device. Theinventive method includes (1) providing in sequence a hole transportlayer, an emissive layer and an electron transport layer to form amedium of organic luminescent layers; and (2) modifying the thicknessdimension of at least one of the layers, each of the layers includingmolecular components corresponding to the layer and having at least twosilicon moieties reactive to a hydroxy functionality and the layerscoupled one to another by Si—O bonds, the modification by reaction ofthe corresponding molecular components one to another to form Si—O bondsbetween the molecular components, and the modification in sequence ofthe provision of the layers.

[0043] In preferred embodiments of this inventive method, at least onesilicon moiety is unreacted after reaction with a hydroxy functionality.In highly preferred embodiments, modification then includes hydrolyzingthe unreacted silicon moiety of one of the molecular components to forma hydroxysilyl functionality and condensing the hydroxysilylfunctionality with a silicon moiety of another molecular component toform a siloxane bond sequence between the molecular components.

[0044] In highly preferred embodiments, the silicon moieties arehalogenated or alkoxylated silane moieties. Such embodiments includemodifying the thickness dimension by hydrolyzing the unreacted silanemoiety of one of the molecular components to form a hydroxysilylfunctionality and condensing the hydroxysilyl functionality with asilane moiety of another molecular component to form a siloxane bondsequence between the molecular components.

[0045] While the organic luminescent conductive layers/medium of thisinvention can be described as having a single organic hole injecting ortransporting layer and a single electron injecting or transportinglayer, modification of each of these layers with respect to dimensionalthickness or into multiple layers, as more specifically described below,can result in further refinement or enhancement of device performance byway of the light emitted therefrom. When multiple electron injecting andtransporting layers are present, the layer receiving holes is the layerin which hole-electron interaction occurs, thereby forming theluminescent or emissive layer of the device.

[0046] The articles/devices of this invention can emit light througheither the cathode or the anode. Where emission is through the cathode,the anode need not be light transmissive. Transparent anodes can beformed of selected metal oxides or a combination of metal oxides havinga suitably high work function. Preferred metal oxides have a workfunction of greater than 4 electron volts (eV). Suitable anode metaloxides can be chosen from among the high (>4 eV) work functionmaterials. A transparent anode can also be formed of a transparent metaloxide layer on a support or as a separate foil or sheet.

[0047] The devices/articles of this invention can employ a cathodeconstructed of any metal, including any high or low work function metal,heretofore taught to be useful for this purpose and as furtherelaborated in that portion of the incorporated patent referenced in thepreceding paragraph. As mentioned therein, fabrication, performance, andstability advantages can be realized by forming the cathode of acombination of a low work function (<4 eV) metal and at least one othermetal. Available low work function metal choices for the cathode arelisted in cols. 19-20 of the aforementioned incorporated patent, byperiods of the Periodic Table of Elements and categorized into 0.5 eVwork function groups. All work functions provided therein are from Sze,Physics of Semiconductor Devices, Wiley, N.Y., 1969, p.366.

[0048] A second metal can be included in the cathode to increase storageand operational stability. The second metal can be chosen from among anymetal other than an alkali metal. The second metal can itself be a lowwork function metal and thus be chosen from the above-referenced listand having a work function of less than 4 eV. To the extent that thesecond metal exhibits a low work function it can, of course, supplementthe first metal in facilitating electron injection.

[0049] Alternatively, the second metal can be chosen from any of thevarious metals having a work function greater than 4 eV. These metalsinclude elements resistant to oxidation and, therefore, those morecommonly fabricated as metallic elements. To the extent the second metalremains invariant in the article or device, it can contribute to thestability. Available higher work function (4 eV or greater) metalchoices for the cathode are listed in lines 50-69 of col. 20 and lines1-15 of col. 21 of the aforementioned incorporated patent, by periods ofthe Periodic Table of Elements and categorized into 0.5 eV work functiongroups.

[0050] As described more fully in U.S. Pat. No. 5,156,918 which isincorporated herein by reference in its entirety, the electrodes and/orsubstrates of this invention have, preferably, a surface with polarreactive groups, such as a hydroxyl (—OH) group. Materials suitable foruse with or as electrodes and/or substrates for anchoring the conductivelayers and luminescent media of this invention should conform to thefollowing requirements: any solid material exposing a high energy(polar) surface to which layer-forming molecules can bind. These mayinclude: metals, metal oxides such as SiO₂, TiO₂, MgO, and Al₂O₃(sapphire), semiconductors, glasses, silica, quartz, salts, organic andinorganic polymers, organic and inorganic crystals and the like.

[0051] Inorganic oxides (in the form of crystals or thin films) areespecially preferred because oxides yield satisfactory hydrophilic metalhydroxyl groups on the surface upon proper treatment. These hydroxylgroups react readily with a variety of silyl coupling reagents tointroduce desired coupling functionalities that can in turn facilitatethe introduction of other organic components.

[0052] The physical and chemical nature of the anode materials dictatesspecific cleaning procedures to improve the utility of this invention.Alkaline processes (NaOH aq.) are generally used. This process willgenerate a fresh hydroxylated surface layer on the substrates while themetal oxide bond on the surface is cleaved to form vicinal hydroxylgroups. High surface hydroxyl densities on the anode surface can beobtained by sonicating the substrates in an aqueous base bath. Thehydroxyl groups on the surface will anchor and orient any of themolecular components/agents described herein. As described more fullybelow, molecules such as organosilanes with hydrophilic functionalgroups can orient to form the conductive layers.

[0053] Other considerations relating to the design, material choice andconstruction of electrodes and/or substrates useful with this inventionare as provided in the above referenced and incorporated U.S. Pat. No.5,409,783 and in particular cols. 21-23 thereof, such considerations aswould be modified by those skilled in the art in accordance with themolecular conductive layers, and assembly methods and objects of thepresent invention.

[0054] The conductive layers and/or organic luminescent medium of thedevices/articles of this invention preferably contain at least twoseparate layers, at least one layer for transporting electrons injectedfrom the cathode and at least one layer for transporting holes injectedfrom the anode. As is more specifically taught in U.S. Pat. No.4,720,432, incorporated herein by reference in its entirety, the latteris in turn preferably at least two layers, one in contact with theanode, providing a hole injecting zone and a layer between the holeinjecting zone and the electron transport layer, providing a holetransporting zone. While several preferred embodiments of this inventionare described as employing at least three separate organic layers, itwill be appreciated that either the layer forming the hole injectingzone or the layer forming the hole transporting zone can be omitted andthe remaining layer will perform both functions. However, enhancedinitial and sustained performance levels of the articles or devices ofthis invention can be realized when separate hole injecting and holetransporting layers are used in combination.

[0055] Porphyrinic and phthalocyanic compounds of the type described incols. 11-15 of the referenced/incorporated U.S. Pat. No. 5,409,783 canbe used to form the hole injecting zone. In particular, thephthalocyanine structure shown in column 11 is representative,particularly where X can be, but is not limited to, analkyltrichlorosilane, alkyltrialkoxysilane, dialkyldialkoxysilane, ordialkyldichlorosilane functionality and where the alkyl and alkoxygroups can contain 1-6 carbon atoms or is hydrogen. Preferredporphyrinic compounds are represented by the structure shown in col. 14and where R, T¹ and T² can be but are not limited to analkyltrichlorosilane, alkyltrialkoxysilane, dialkyldialkoxysilane, ordialkyldichlorosilane functionality and where the alkyl and alkoxygroups contain 1-6 carbon atoms or is hydrogen. (See, also, FIGS. 1A and1B, herein.) Preferred phthalocyanine- and porphyrin-based holeinjection agents include silicon phthalocyanine dichloride and5,10,15,20-tetraphenyl-21H,23H-porphine silicon (IV) dichloride,respectively.

[0056] The hole transporting layer is preferably one which contains atleast one tertiary aromatic amine, examples of which are as described inFIGS. 2A-2C and Examples 1 and 2. Other exemplary arylamine corestructures are illustrated in U.S. Pat. No. 3,180,730, which isincorporated herein by reference in its entirety, where the corestructures are modified as described herein. Other suitabletriarylamines substituted with a vinyl or vinylene radical and/orcontaining at least one active hydrogen containing group are disclosedin U.S. Pat. Nos. 5,409,783, 3,567,450 and 3,658,520. These patents areincorporated herein by reference in their entirety and the corestructures disclosed are modified as described herein. In particular,with respect to the arylamines represented by structural formulas XXIand XXIII in cols. 15-16 of U.S. Pat. No. 5,409,703, R²⁴, R^(25,) R²⁶,R²⁷, R³⁰, R³¹ and R³² can be an alkyltrichlorosilane,alkyltrialkoxysilane, dialkyldialkoxysilane, or dialkyldichlorosilanefunctionality where the alkyl and alkoxy groups can contain 1-6 carbonatoms or is hydrogen.

[0057] Molecular components of this invention comprising emissive agentsand/or the emissive layer include those described herein in FIGS. 3A-3Cand Example 5. Other such components/agents include various metalchelated oxinoid compounds, including chelates of oxine (also commonlyreferred to as 8-quinolinol or 8-hydroxyquinoline), such as thoserepresented by structure III in col. 8 of the referenced andincorporated U.S. Pat. No. 5,409,783, and where Z² can be but is notlimited to an alkyltrichlorosilane, alkyltrialkoxysilane,dialkyldialkoxysilane, or dialkyldichlorosilane functionality and wherethe alkyl and alkoxy groups can contain 1-6 carbon atoms or is hydrogen.Other such molecular components/emissive agents include thequinolinolato compounds represented in cols. 7-8 of U.S. Pat. 5,151,629,also incorporated herein by reference in its entirety, where a ringsubstituent can be but is not limited to an alkyltrichlorosilane,alkyltrialkoxysilane, dialkyldialkoxysilane, or dialkyldichlorosilanefunctionality and where the alkyl and alkoxy groups can contain 1-6carbon atoms or is hydrogen. In a similar fashion, the dimethylidenecompounds of U.S. Pat. No. 5,130,603, also incorporated herein byreference in its entirety, can be used, as modified in accordance withthis invention such that the aryl substituents can include analkyltrichlorosilane, alkyltrialkoxysilane, dialkyldialkoxysilane, ordialkyldichlorosilane functionality and where the alkyl and alkoxygroups can contain 1-6 carbon atoms or is hydrogen.

[0058] Other components which can be used as emissive agents includewithout limitation anthracene, naphthalene, phenanthrene, pyrene,chrysene, perylene and other fused ring compounds, or as provided incol. 17 of the previously referenced and incorporated U.S. Pat. No.5,409,783, such compounds as modified in accordance with this inventionand as more fully described above. Modifiable components also includethose described in U.S. Pat. Nos. 3,172,862, 3,173,050 and 3,710,167—allof which are incorporated herein by reference in their entirety.

[0059] Molecular components which can be utilized as electron injectingor electron transport agents and/or in conjunction with an electroninjection or electron transport layer are as described in FIGS. 4A-4Cand Examples 3(a)-(d) and 4. Other such components include oxadiazolecompounds such as those shown in cols. 12-13 of U.S. Pat. No. 5,276,381,also incorporated herein by reference in its entirety, as such compoundswould be modified in accordance with this invention such that the phenylsubstituents thereof each include an alkyltrichlorosilane,alkyltrialkoxysilane, dialkyldialkoxysilane, or dialkyldichlorosilanefunctionality and where the alkyl and alkoxy groups can contain 1-6carbon atoms or is hydrogen. Likewise, such components can be derivedfrom the thiadiazole compounds described in U.S. Pat. No. 5,336,546which is incorporated herein by reference in its entirety.

[0060] As described above, inorganic silicon moieties can be used inconjunction with the various molecular components, agents, conductivelayers and/or capping layers. In particular, silane moieties can be usedwith good effect to impart mechanical, thermal, chemical and/orphotochemical stability to the luminescent medium and/or device. Suchmoieties are especially useful in conjunction with the methodologydescribed herein. Degradation is minimized until further syntheticmodification is desired. Hydrolysis of an unreacted silicon/silanemoiety provides an Si—OH functionality reactive with a silicon/silanemoiety of another component, agent and/or conductive layer. Hydrolysisproceeds quickly in quantitative yield, as does a subsequentcondensation reaction with an unreacted silicon/silane moiety of anothercomponent to provide a siloxane bond sequence between components, agentsand/or conductive layers.

[0061] In general, the molecular agents/components in FIGS. 1-4 can beprepared with a lithium or Grignard reagent using synthetic techniquesknown to one skilled in the art and subsequent reaction with halosilaneor alkoxysilane reagents. Alternatively, unsaturated olefinic oracetylenic groups can be appended from the core structures using knownsynthetic techniques. Subsequently, halosilane or alkoxysilanefunctional groups can be introduced using hydrosilation techniques, alsoknown to one skilled in the art. Purification is carried out usingprocedures appropriate for the specific target molecule.

[0062] It has been observed previously that the performancecharacteristics of electroluminescent articles of the type describedherein can be enhanced by the incorporation of a layer having adielectric function between the anode and, for instance, a holetransport layer. Previous studies show that the vapor deposition of thinlayers of LiF onto an anode before deposition of the other layer(s)improves performance in the areas of luminescence and quantumefficiency. However, this technique is limited in that the deposited LiFfilms are rough, degrade in air and do not form comformal, pinhole-freecoatings.

[0063] The present invention is also directed to the application ofself-assembly techniques to form layers which cap an electrode, providedielectric and other functions and/or enhance performance relative tothe prior art. Such capping layers are self-assembled films which areconformal in their coverage, can have dimensions less than one nanometerand can be deposited with a great deal of control over the total layerthickness. Accordingly, the present invention also includes anelectroluminescent article or device which includes (1) an anode, (2) atleast one molecular capping layer coupled to the anode withsilicon-oxygen bonds, with each capping layer coupled one to anotherwith silicon-oxygen bonds, (3) a plurality of molecular conductivelayers, with one of the layers coupled to the capping layer withsilicon-oxygen bonds and each conductive layer coupled one to anotherwith silicon-oxygen bonds, and (4) a cathode in electric contact with aconductive layer. Likewise, and in accordance with this invention, thecapping layer can be deposited on a conductive layer and/or otherwiseintroduced so as to be adjacent to a cathode, to enhance overallperformance.

[0064] More generally and within the scope of this invention, the anodeis separated from the cathode by an organic luminescent medium. Theanode and cathode are connected to an external power source byconductors. The power source can be a continuous direct, alternating orintermittent current voltage source. A convenient conventional powersource, including any desired switching circuitry, which is capable ofpositively biasing the anode with respect to the cathode, can beemployed. Either the anode or cathode can be at ground potential.

[0065] In preferred embodiments, each conductive and/or capping layerhas molecular components, and each molecular component has at least twosilicon moieties. In highly preferred embodiments, each such conductiveand/or capping component is a halogenated or alkoxylated silane, andsilicon-oxygen bonds are obtainable from the condensation of the silanemoieties with hydroxy functionalities. Without limitation, a preferredcapping material is octachlorotrisiloxane. The anode and cathode can bechosen and/or constructed as otherwise described herein.

[0066] In part, the present invention is a method of using moleculardimension to control the forward light output of an electroluminescentdevice. The inventive method includes (1) providing an electrode and amolecular layer thereon, the layer coupled to the electrode with firstmolecular components having at least two silicon moieties reactive to ahydroxy functionality; and (2) modifying the thickness of the layer byreacting the molecular components with second components to form asiloxane bond sequence between the first and second molecularcomponents, the second molecular components having at least two siliconmoieties also reactive to a hydroxy functionality.

[0067] In preferred embodiments of this inventive method, at least onesilicon moiety is unreacted after reaction with a hydroxy functionality.In highly preferred embodiments, the modification further includeshydrolyzing an unreacted silicon moiety of one of the molecularcomponents to form a hydroxysilyl functionality and condensing thehydroxysilyl functionality with a silicon moiety of a third molecularcomponent to form a siloxane bond sequence between the second and thirdmolecular components. In highly preferred embodiments, the siliconmoieties are halogenated or alkoxylated silane moieties.

[0068] In part, the present invention also includes anyelectroluminescent article for generating light upon application of anelectrical potential across two electrodes. Such an article includes anelectrode having a surface portion and a molecular layer coupled and/orcapped thereon. The layer includes molecular components, and eachcomponent has at least two silicon moieties. The layer is coupled to theelectrode with silicon-oxygen bonds. In preferred embodiments, eachsilicon moiety is a halogenated silane, and silicon-oxygen bonds areobtained from a condensation reaction. Likewise, and without limitation,the electrode has a substrate with a hydroxylated surface portiontransparent to near-IR and visible wavelengths of light. Such a layercan be utilized to cap the electrode and/or enhance performance asotherwise described herein. More generally, in such an article or anyother described herein, the luminescent medium can be constructed usingeither the self-assembly techniques described herein or the materialsand techniques of the prior art.

[0069] The electroluminescent devices and related methods of thisinvention can demonstrate various interlayer/interfacial phenomenathrough choice of layer/molecular components and design of the resultingelectroluminescent medium. As a point of reference, a number of cathodeand anode interfacial structures can enhance charge injection, hencedevice performance. For instance, with vapor-deposited, anode/TPD(N-N′-diphenyl-N-N′-bis(3-methylphenyl)-(1-1′-biphenyl)-4-4′-diamine)/Alq(tris(quinoxalinato)Al(III))/cathode devices of the prior art, adramatic increase in light output and quantum efficiency occurs whenÅ-scale LiF or CsF layers are interposed between the cathode andelectron transport layer (ETL). Such thin dielectric layers are thoughtto lower the Al work function, thus reducing the effective electroninjection barrier (energy level offset between the Alq LUMO and the AlFermi level).

[0070] In contrast, modification of the ITO anode—hole transport layer(HTL) interface is somewhat more controllable, although similarmechanistic uncertainties pertain. Thus, a variety of ITOfunctionalization approaches produce phenomenologically similar effects,although less dramatic than those observed for the interposition ofalkali fluoride at Al cathodes. These approaches include deposition ontoITO of nanoscale layers of various organic acids, copper phthalocyanine,or thicker (30-100 nm) layers of polyaniline or polythiophene (PEDOT),all resulting in somewhat enhanced luminous performance. Explanationsfor these phenomena are diverse, ranging from altering interfacialelectric fields, balancing electron/hole injection fluence, confiningelectrons in the emissive layer, reducing injected chargeback-scattering, and moderating anode Fermi level-HTL HOMO energeticdiscontinuities. This diversity of proposed mechanisms accuratelyreflects the complexity of interactions at OLED interfaces and, in manycases, the lack of necessary microstructural information.

[0071] In a departure from the prior art, the present invention can beconsidered in the context of one or more structural relationshipsbetween an OLED anode and/or its associated organic layers. Withoutrestriction to any one theory or mode of operation, moderation of thesurface energy mismatch can be effected at a hydrophilic oxideanode-hydrophobic HTL interface, as demonstrated below using nanoscopicself-assembled silyl group functionalized amine components (see, forexample, FIG. 11A and 1-4 TAA layers; 11 Å/layer). Promotinganode/ITO-HTL physical cohesion significantly enhances luminousperformance and durability. Furthermore, as relates to another aspect ofthis invention, a silyl-group functionalized, crosslinkable amine layerhaving the core amine structure of the HTL component, TPD, (TPD-Si₂,FIG. 11A) significantly improves ITO/TPD/Alq/Al device performance andthermal durability (one metric of device stability) to an extentsurprising, unexpected and unattainable with other anodefunctionalization structures. In contrast thereto, the commonly usedcopper phthalocyanine (Cu(Pc); FIG. 11A) anode functionalization layeractually templates crystallization of overlying TPD films at modesttemperatures (Example 13 and FIG. 12D), consistent with the thermalinstability of many Cu(Pc)-buffered OLED devices of the prior art.

[0072] Accordingly, in its broader respects, the present inventioncontemplates a method of using an amine molecular component to enhancehole injection across the electrode-organic interface of a lightemitting diode device. The inventive method includes (1) providing ananode; and (2) incorporating an electroluminescent medium adjacent theanode, the medium including but not limited to a molecular layer,coupled to the anode, of amine molecular components substituted with atleast one silyl group, and thereon a hole transport layer of molecularcomponents having the amine structure of the aforementioned molecularlayer components. The molecular layer can have at least one of anarylamine component and an arylalkylamine component, including but notlimited to those monoarylamine, diarylamine and triarylamine componentsdescribed in the aforementioned and incorporated U.S. Pat. No.5,409,783, modified and/or silyl-functionalized as provided herein.Other suitable arylamine and/or arylalkylamine structures are disclosedin U.S. Pat. Nos. 3,180,730, 3,567,450 and 3,658,520, each of which isincorporated herein in its entirety, such structures as can also bemodified to provide silyl-functionality in accordance herewith.Likewise, a combination of such silyl-substituted components can beemployed with beneficial effect.

[0073] In preferred embodiments of this inventive method, theaforementioned amine molecular layer components arealkylsilyl-substituted compounds of the type illustrated in FIGS. 2A and2C. In highly preferred embodiments, such components include thealkylsilyl-substituted TAA and alkylsilyl substituted TPD compoundsprepared as described herein. Regardless, such a molecular layer can bespin-coated on the anode surface or self-assembled, as described morefully above, to provide silicon-oxygen bonds therewith. A plurality ofsuch molecular layers can be coupled successively on an anodesurface—each layer coupled one to another with silicon-oxygen bonds—toimprove structural stability and enhance device performance. Asdescribed herein and with reference to several of the followingexamples, hole injection can be enhanced by choice of a molecular layerwith components having a structural relationship with those arylamine orarylalkylamine components of the hole transport layer. In preferredembodiments, such enhancement can be achieved through use of asilyl-functionalized TPD layer in conjunction with a TPD hole transportlayer.

[0074] As such, the present invention also includes an organicelectroluminescent device for generating light upon application of anelectrical potential cross to electrodes. Such a device includes (1) ananode; (2) at least one molecular layer, coupled to the anode, of one ormore of the aforementioned amine molecular components substituted withat least one silyl group; (3) a conductive layer of molecular componentshaving the amine structure; and (4) a cathode in electrical contact withthe anode. A preferred conductive layer includes a hole transport layercomprising components of the prior art incorporated herein by reference,or modified as described above. Preferred molecular layer components arealkylsilyl-substituted compounds of the type illustrated in FIGS. 2A and2C, in particular silyl-functionalized TAA and TPD. In light of theaforementioned structural relationships and associated methodologies, apreferred conductive layer of such a device is a TPD hole transportlayer, such a layer substantially without crystallization upon annealingand/or at device operation temperatures when used in conjunction with amolecular layer of components having the same or a structurally similaramine structure.

[0075] As demonstrated herein, hydrophobic amine HTL—hydrophilic anodeintegrity is a factor in OLED performance; poor physical cohesioncontributes to inefficient hole injection and ultimately, devicefailure. Enhanced performance can be achieved through use of molecularlayer structures which maximize interfacial cohesion and chargetransport. With reference to one preferred embodiment, a convenientlyapplied, spincoated silyl (Si) functionalized TPD analogue, TPD-Si₂(FIGS. 11A-B), structurally similar to the overlying HTL, hencewell-suited to stabilizing the interface, undergoes rapid crosslinkingupon spincoating from solution and subsequent thermal curing to form adense, robust siloxane matrix with imbedded TPD hole-transportcomponents. The thickness of these layers (˜40 nm) was determined byspecular X-ray reflectivity on samples deposited via identicaltechniques on single-crystal silicon. The RMS roughness of the TPD-Si₂molecular layer films on ITO substrates of 30 Å RMS roughness is 8-12 Åby contact mode AFM. Crosslinked TPD-Si₂ films exhibit high thermalstability, with only 5% weight loss observed up to 400° C. by TGA,indicating substantial resistance to thermal degradation. Furthermore,cyclic voltammetry of 40 nm TPD-Si₂ films on ITO electrodes indicatesthat they support facile hole transport and are electrochemicallystable.

[0076] The densely crosslinked nature of TPD-Si₂ molecular layer filmsis evident in the relatively large separation of oxidative and reductivepeaks (200 mV), suggesting kinetically hindered oxidation/reductionprocesses with retarded counterion mobility. P. E. Smolenyak, E. J.Osburn, S. -Y. Chen, L. -K. Chau, D. F. O'Brian, N. R. Armstrong,Langmuir 1997, 21, 6568. That TPD-Si₂ film coverage on ITO is conformaland largely pinhole-free is supported by studies using a previouslydescribed ferrocene probe technique. W. Li, Q. Wang, J. Cui, H. Chou, T.J. Marks, G. E. Jabbour, S. E. Shaheen, B. Kippelen, N. Pegyhambarian,P. Dutta, A. J. Richter, J. Anderson, P. Lee, N. Armstrong, Adv. Mater.1999, 11, 730. The lack of significant current flow near the formalpotential for ferrocene oxidation at a TPD-Si₂-coated ITO workingelectrode indicates suppression of ferrocene oxidation, consistent withlargely pinhole-free surface coverage. G. Inzelt, ElectroanalyticalChemistry, Vol. 18, Marcel Dekker, New York, 1994, p. 89.

EXAMPLES OF THE INVENTION

[0077] The following non-limiting examples and data illustrate variousaspects and features relating to the articles/devices and/or methods ofthe present invention, including the assembly of a luminescent mediumhaving various molecular components/agents and/or conductive layers, asare available through the synthetic methodology described herein. Incomparison with the prior art, the present methods and articles/devicesprovide results and data which are surprising, unexpected and contraryto the prior art. While the utility of this invention is illustratedthrough the use of several articles/devices and molecularcomponents/agents/layers which can be used therewith, it will beunderstood by those skilled in the art that comparable results areobtainable with various other articles/devices andcomponents/agents/layers, as are commensurate with the scope of thisinvention.

Example 1

[0078]

[0079] Synthesis of a Silanated Hole Transport Agent [1]. With referenceto reaction scheme, above, hole transport components, agents and/orlayers can be prepared, in accordance with this invention and/or for usein conjunction with light-emitting diodes and other similarelectroluminescent devices. Accordingly, 500 mg. (1.0 mmole) oftrisbromophenylamine (Aldrich Chemical Company) was dissolved in 30 mlof dry dimethoxyethane (DME). This solution was cooled to −45° C. and1.2 ml (3.3 mmole) of a 2.5 M solution of n-butyl lithium in hexane wasadded to the reaction mixture. The entire mixture was then slowly warmedto 20° C. After stirring at 20° C. for an additional hour, the solventwas removed in vacuo. The resulting white precipitate was washed (3×20ml) with dry pentane and redissolved in 30 ml dry DME. This solution wassubsequently poured into 10 ml (87 mmole) of silicon tetrachloride at arate of 1 ml/min. The entire reaction mixture was then refluxed for twohours. The resulting supernatent was separated from the precipitate, andthe solvent again removed in vacuo yielding a green-brown residue. Awhite solid was obtained from this residue upon sublimation at 10⁻⁶torr. Characterization: ¹H NMR (600 MHz, C₆D₆, 20° C.): δ7.07 (d, 6H,Ar—H); δ7.05 (d, 6H, Ar—H); EI-MS (m/z): 645 (M+).

Example 2

[0080] With reference to FIGS. 2A-2C and the representative arylaminesprovided therein, other hole transport agents and/or layers of thisinvention can be obtained by straightforward application of thesilanation procedure described above in Example 1, with routinesynthetic modification(s) and optimization of reaction conditions aswould be well-known to those skilled in the art and as required by theparticular arylamine. Likewise, preliminary halogenation/bromination canbe effected using known synthetic procedures. Alternatively, thearylamines of FIGS. 2A-2C and other suitable substrates can be preparedusing other available synthetic procedures to provide multiple silanereaction centers for use with the self-assembly methods andlight-emitting diodes of this invention. Core molecular substrates ofthe type from which the arylamines of FIGS. 2A-2C can be prepared aredescribed by Strukelji et al. in Science, 267, 1969 (1995), which isincorporated herein by reference in its entirety.

Example 3

[0081] Synthesis of a Silanated Electron Transport Agent. With referenceto Examples 3(a)-(d) and corresponding reaction schemes, below, electrontransport agents and/or layers can be prepared, in accordance with thisinvention and/or for use in conjunction with light-emitting diodes andother similar electroluminescent devices.

Example 3a

[0082] Synthesis of 4′-Bromo-2-(4-bromobenzoyl)acetophenone [2]. In a1-liter three neck round bottom flask, 43 g (0.2 mol) methyl4-bromobenzoic acid and 17.6 g (0.4 mol) sodium hydride were dissolvedin 200 ml dried benzene and heated to 60° C. Next, 39.8 g (0.2 mol)4-bromoacetophenone in 100 ml dry benzene was slowly added through adropping funnel, and 1 ml methanol was added to the flask to initiatethe reaction. After the mixture was refluxed overnight, the reaction wasquenched by adding methanol and pouring it into ice water. The pH of themixture was brought down to 7.0 using 5 N sulfuric acid. A solid wascollected, washed with water, and recrystallized from benzene to give alight yellow product. Characterization. Yield: 30.3 g (40%). ¹H NMR (300MHz, CDCl₃, 20° C., δ): 7.84 (d, 4H, ArH); 7.62 (d, 4H, ArH); 6.77(s,2H, CH₂). EI-MS: 382(M+), 301, 225, 183, 157.

Example 3b

[0083] Synthesis of 3,5-Bis(4-bromophenyl)isoxazole [3]. In a 250 mlround bottom flask, 4 g (10.4 mmol) of [2] was dissolved in 100 ml drydioxane and heated to reflux, then 3.0 g (43.2 mmol) hydroxylaminehydrogen chloride in 10 ml water and 5 ml (25 mmol) 5 N NaOH was thendropped into the refluxing mixture. After 12 hours, the reaction mixturewas cooled down to room temperature, and the solvent was removed invacuo. The product was recrystallized from ethanol. Characterization.Yield: 3.41 g (85%). M.P. 218.5-219.5° C. ¹H NMR (300 MHz, CDCl₃, 20°C., δ): 7.78(d, 2H, ArH), 7.74 (d, 2H, Ar′H), 7.66 (d, 2H, ArH), 7.6 (d,2H, Ar′H), 6.82 (s, 1H, isoxazole proton). EI-MS: 379(M+), 224, 183,155.

Example 3c

[0084] Synthesis of 3,5-Bis(4-allylphenyl)isoxazole [4]. In a 250 mlthree-neck round bottom flask, 3.77 g (10 mmol) of [3], 460 mg. (0.4mmol) tetrakis-(triphenylphosphine) palladium, and 7.28 g (22 mmol)tributylallyltin were dissolved in 100 ml. dried toluene and degassedwith nitrogen for 30 min. The mixture was heated to 100° C. for 10 h,then cooled down to room temperature. Next, 50 ml. of a saturatedaqueous ammonium fluoride solution was subsequently added to themixture. The mixture was extracted with ether, and the combined organiclayer was washed by water, then brine, and finally dried over sodiumsulfate. The solvent was removed in vacuo. The residue was purified bycolumn chromatography. (first, 100% hexanes, then chloroform:hexanes[80:20]). Characterization. Yield: 1.55 g (57%). 1H NMR (300 MHz, CDCl₃,20° C., δ): 7.78(d, 2H, ArH), 7.74 (d, 2H, Ar′H), 7.34 (d, 2H, ArH),7.30 (d, 2H, Ar′H), 6.78 (s, 1H, isoxazole proton), 5.96 (m, 2H, alkeneH), 5.14 (d, 4H, terminal alkene H), 3.44 (d, 4H, methylene group).EI-MS: 299(M+), 258, 217.

Example 3d

[0085] Synthesis of 3,5-Bis(4-(N-trichlorosilyl)propylphenyl)isoxazole[5]. To 2 ml of THF was added 5 mg of [4], 3.4 μl of HSiCl₃ and 0.8 mg.of H₂PtCl₆ were added to 2 ml of THF. The reaction was heated at 50° C.for 14 h. The solvent was then removed in vacuo. A white solid wasobtained from this residue upon sublimation at 10⁻⁶ torr.Characterization. ¹H NMR (300 MHz, d⁸-THF, 20° C., δ): 7.72(d, 2H, ArH),7.68 (d, 2H, Ar′H), 7.36 (d, 2H, ArH), 7.32 (d, 2H, Ar′H), 6.30(s, 1H,isoxazole); 2.52(t, 2H, CH); 1.55 (m, 4H, CH₂); 0.85 (t, 6H, CH₃).

Example 4

[0086] With reference to FIGS. 4A-4C and the representative heterocyclesprovided therein, other electron transport agents and/or layers of thisinvention can be obtained by straight-forward application of thesilanation procedure described above in Example 3, with routinesynthetic modification(s) and optimization of reaction conditions aswould be well-known to those skilled in the art and as required by theparticular heterocyclic substrate. Preliminary halogenation/brominationcan be effected using known synthetic procedures or through choice ofstarting materials enroute to a given heterocycle. Alternatively, theheterocycles of FIGS. 4A-4C and other suitable substrates can beprepared using other available synthetic procedures to provide multiplesilane reaction centers for use with the self-assembly methods andlight-emitting diodes of this invention. Core molecular substrates ofthe type from which the heterocycles of FIGS. 4A-4C can be prepared arealso described by Strukelji et al. in Science, 267, 1969 (1995).

Example 5

[0087] With reference to FIGS. 3A-3C and the representative chromophoresprovided therein, emissive agents and/or layers, in accordance with thisinvention, can be obtained by appropriate choice of starting materialsand using halogenation and silanation procedures of the type describedin Examples 1-4, above. Alternatively, other chromophores can besilanated using other available synthetic procedures to provide multiplesilane reaction centers for use with the self-assembly methods andlight-emitting diodes of this invention. Regardless, in accordance withthis invention, such emissive agents or chromophores can be used foremission of light at wavelengths heretofore unpractical or unavailable.Likewise, the present invention allows for the use of multiple agents orchromophores and construction of an emissive layer or layers such that acombination of wavelengths and/or white light can be emitted.

Example 6

[0088] Examples 6(a)-6(c) together with FIG. 6 illustrate thepreparation of other molecular components which can be used inaccordance with this invention.

Example 6a

[0089] Synthesis of Tertiary Arylamine [6]. Together, 14.46 g (20 mmole)of tris(4-bromophenyl)amine and 500 ml of dry diethyl ether were stirredat −78° C. under a nitrogen atmosphere. Next, 112.5 ml of a 1.6 Mn-butyllithium solution in hexanes was slowly added to the reactionmixture over 1.5 hours. The reaction was then warmed to ˜10° C. andstirred for an additional 30 minutes. The reaction was then cooled downagain to −78° C. before the addition of 22 g (0.5 mole) of ethyleneoxide. The mixture was stirred and slowly warmed to room temperatureover 12 hours. Next, 2 ml of a dilute NH₄Cl solution was then added tothe reaction mixture. The solvent was evaporated under vacuum yielding alight green solid. The product was purified using column chromatography.The column was first eluted with chloroform and then with MeOH:CH₂Cl₂(5:95 v/v). The resulting light gray solid was recrystallized usingchloroform to give 1.89 g. Yield: 25%. ¹H NMR (δ, 20° C., DMSO): 2.65(t,6H), 3.57 (q,6H), 4.64 (t,3H), 6.45 (d,6H), 7.09 (d,6H). EI-MS: 377(M⁺), 346 (M⁺−31), 315 (M⁺−62). HRMS: 377.2002. calcd; 377.1991. Anal.Calculated for C₂₄H₂₇NO₃; C, 76.36; H, 7.21; N, 3.71. Found: C, 76.55;H, 7.01; N, 3.52.

Example 6b

[0090] Synthesis of Tosylated Arylamine [7]. A pyridine solution oftosyl chloride (380 mg in 5 ml) was added over 5 minutes to a pyridinesolution of [6] (500 mg in 10 ml, from Example 6a) cooled to 0° C. Themixture was stirred for 12 hours, then quenched with water and extractedwith chloroform. The organic extract was washed with water, 5% sodiumbicarbonate, and dried with magnesium sulfate. After filtration, thechloroform solution was then evaporated to dryness under vacuum andpurified using column chromatography. The column was first eluted withhexane:CHCl₃ (1:2 v/v) yielding [7]. ¹H NMR (300 MHz, δ, 20° C., CDCl₃):2.45 (s,3H), 2.90 (t,6H), 3.02 (t,3H), 3.70 (t,3H), 4.19 (t,6H), 6.92(d,2H), 6.98 (d,4H), 7.00 (d,4H), 711 (d,2H), 7.32 (d,2H), 7.77 (d,2H).

Example 6c

[0091] Synthesis of Tosylated Arylamine [8]. Continuing thechromatographic procedure similar for 2 (from Example 6b)but changingthe eluting solvent to 100% CHCl₃ yielded [8]. ¹H NMR (300 MHz, δ, 20°C., CDCl₃): 2.44 (s,6H), 2.91 (t,3H), 3.02 (t,6H), 3.70 (t,6H), 4.19(t,3H), 6.92 (d,4H), 6.98 (d,2H), 7.00 (d,2H), 7.11 (d,4H), 7.32 (d,4H),7.77 (d,4H).

Example 7

[0092] Using the arylamines of Examples 6 and with reference to FIG. 7,an electroluminescent article/device also in accordance with thisinvention is prepared as described, below. It is understood that thearylamine component can undergo another or a series of reactions with asilicon/silane moiety of another molecular component/agent to provide asiloxane bond sequence between components, agents and/or conductivelayers. Similar electroluminescent articles/devices and conductivelayers/media can be prepared utilizing the various other molecularcomponents/agents and/or layers described above, such as in Examples 1-5and FIGS. 1-4, in conjunction with the synthetic modifications of thisinvention and as required to provide the components with the appropriatereactivity and functionality necessary for the assembly method(s)described herein.

Example 7a

[0093] This example of the invention shows how slides can beprepared/cleaned prior to use as or with electrode materials. Anindium-tin-oxide (ITO)-coated soda lime glass (Delta Technologies) wasboiled in a 20% aqueous solution of ethanolamine for 5 minutes, rinsedwith copious amounts of distilled water and dried for 1 hour at 120° C.;alternatively and with equal effect, an ITO-coated soda lime glass(Delta Technologies) was sonicated in 0.5M KOH for 20 minutes, rinsedwith copious amounts of distilled water and then ethanol, and dried for1 hour at 120° C.

Example 7b

[0094] Electroluminescent Article Fabrication and Use. The freshlycleaned ITO-coated slides were placed in a 1% aqueous solution of3-aminopropyltrimethoxysilane and then agitated for 5 minutes. Thesecoated slides were then rinsed with distilled water and cured for 1 hourat 120° C. The slides were subsequently placed in a 1% toluene solutionof [7] (or [8] from Example 6) and stirred for 18 hours under ambientconditions. Afterwards, the slides were washed with toluene and curedfor 15 minutes at 120° C. AlQ₃ (or GaQ₃; Q=quinoxalate) was vapordeposited on top of the amine-coated slides. Finally, 750-1000 Å ofaluminum was vapor deposited over the metal quinolate layer. Wires wereattached to the Al and ITO layers using silver conducting epoxy(CircuitWorks™), and when a potential (<7V) was applied, red, orange,and/or green light was emitted from the device.

Example 8

[0095] One or more capping layers comprising Cl₃SiOSiCl₂OSiCl₃ aresuccessively deposited onto clean ITO-coated glass where hydrolysis ofthe deposited material followed by thermal curing/crosslinking in air at125° C. yields a thin (˜7.8 Å) layer of material on the ITO surface.X-ray reflectivity measurements indicate that the total film thicknessincreases linearly with repeated layer deposition, as seen in FIG. 8.Other molecular components can be used with similar effect. Suchcomponents include, without limitation, the bifunctional siliconcompounds described in U.S. Pat. No. 5,156,918, at column 7 andelsewhere therein, incorporated by reference herein in its entirety.Other useful components, in accordance with this invention include thosetrifuctional compounds which cross-link upon curing. As would be wellknown to those skilled in the art and made aware of this invention, suchcomponents include those compounds chemically reactive with both theelectrode capped and an adjacent conductive layer.

Example 9

[0096] Cyclic voltametry measurements shown in FIG. 9 using aqueousferri/ferrocyanide show that there is considerable blocking of theelectrode after the deposition of just one layer of the self-assembledcapping material specified in Example 8. Other molecular componentsdescribed, above, show similar utility. Almost complete blocking, asmanifested by the absence of pinholes, is observed after application ofthree layers of capping material.

Example 10

[0097] Conventional organic electroluminescent devices consisting of TPD(600 Å)/Alq (600 Å)/Mg (2000 Å) were vapor-deposited on ITO substratesmodified with the capping material specified in Example 8. FIGS. 10A-Cshow the behavior of these devices with varying thickness of theself-assembled capping material. These results show that such a materialcan be used to modify forward light output and device quantumefficiency. For a device with two capping layers, higher currentdensities and increased forward light output are achieved at lowervoltages, suggesting an optimum thickness of capping material can beused to maximize performance of an electroluminescent article.

Example 11

[0098] This example illustrates how a capping material can be introducedto and/or used in the construction of an electroluminescent article.ITO-coated glass substrates were cleaned by sonication in acetone for 1hour followed by sonication in methanol for 1 hour. The dried substrateswere then reactively ion etched in an oxygen plasma for 30 seconds.Cleaned substrates were placed in a reaction vessel and purged withnitrogen. A suitable silane, for instance a 24 mM solution ofoctachlorotrisiloxane in heptane, was added to the reaction vessel in aquantity sufficient to totally immerse the substrates. (Other suchcompounds include those described in Example 8). Substrates were allowedto soak in the solution under nitrogen for 30 minutes. Following removalof the siloxane solution the substrates were washed and sonicated infreshly distilled pentane followed by a second pentane wash undernitrogen. Substrates were then removed from the reaction vessel washedand sonicated in acetone. Substrates were dried in air at 125° C. for 15minutes. This process can be repeated to form a capping layer ofprecisely controlled thickness.

Example 12

[0099] The stability of device-type TPD-Si₂ molecular layer/TPD holetransport layer interfaces under thermal stress (one measure ofdurability) was investigated by annealing ITO/TPD-Si₂ (40 nm)/TPD (100nm) bilayers at 80° C. for 1.0 h. The optical image of the annealed TPDfilm shows no evidence of TPD de-wetting/decohesion (FIG. 12A),indicating that the ITO-TPD surface energy mismatch is effectivelymoderated by the interfacial TPD-Si₂ molecular layer. In contrast, thebare ITO/TPD interface exhibits catastrophic de-wetting/de-cohesionunder identical thermal cycling (FIG. 12B), visible even under a layerof Alq). Despite seemingly similar cohesive effects for both TAA andTPD-Si₂ as interfacial buffer layers, it is reasonable to suggest thatthe interfacial cohesion between TPD-Si₂ and TPD is greater, givencloser structural similarity, evidenced by comparing advancing aqueouscontact angles: values for bare ITO, silyl-functionalized TAA, silyl(Si₂) functionalized TPD and TPD film surfaces are 0°, 45°, 70°, and 85°respectively, indicating a closer surface energy match at TPD-TPD-Si₂interfaces.

Example 13

[0100] Speculation that one role of Cu(Pc) in enhancing OLED performancemight be via the above adhesion mechanism led to parallel thermalstudies. In contrast to a preferred alkylsilyl-substituted arylamineTPD-Si₂, Cu(Pc)-buffered ITO does not prevent TPD de-cohesion uponheating to temperatures near/above the TPD glass transition temperature(T_(g)). FIG. 12D illustrates the morphology of a 100 nm TPD film on 10nm Cu(Pc) following heating at 80° C. It is clearly seen that thermalannealing induces TPD crystallization on the Cu(Pc) film surface(visible even under a layer of Alq), yielding star-shaped dendriticcrystallites (as-deposited TPD films on Cu(Pc) are smooth andfeatureless, FIG. 12C). It is likely that such Cu(Pc)-nucleatedcrystallization occurs during localized heating in operating OLEDs andcontributes to observed device instability.

Example 14

[0101] Device characteristics employing spincoated TAA, TPD-Si₂, andvapor-deposited Cu(Pc) hole injection layer/adhesion layers in OLEDshaving the structure ITO/interlayer/TPD/Alq/Al are compared in FIG. 13.Versus the bare ITO system, all of the molecular/bufferlayer-incorporated devices exhibit higher light output, enhanced quantumefficiencies, and lower turn-on voltages. Note that a preferred TPD-Si₂component layer affords ˜15,000 cd/m² of maximum light output, which is10-100× greater than the bare ITO-based device. Similar increases inITO/TPD/Alq/Al-type device performance with electrode functionalizationhave only been reported previously for LiF or CsF-modified Al cathodes,via what remains an unresolved mechanism. It is widely accepted thatconventional ITO/TPD/Alq/Al heterostructures are electron-limited due tothe low Alq electron mobility and Alq LUMO-Al Fermi level energeticmismatch, thus raising the question of why TPD-Si₂ anode modificationproduces similar effects. It is suggested that under conditions ofanode-HTL surface energy mismatch and poor anode-HTL cohesion (anunmodified device of prior art), non-ohmic contacts dominate devicebehavior, resulting in significant hole injection barriers typical ofpoor electrode-organic contact.

Example 15

[0102] With reference to FIG. 14, TAA and TPD-Si₂ interfacial/molecularcomponent layers are significantly more effective injection structuresthan conventional Cu(Pc) layers. The maximum light output for aCu(Pc)-based device is ˜1500 cd/m² at 25 V, while that of a TAA-baseddevice is 2600 cd/m², and at a much lower bias voltage (16 V). Theexternal quantum efficiency of the Cu(Pc)-based device also falls wellbelow those based on TAA and TPD-Si₂ anode layers: the maximum quantumefficiency for Cu(Pc) is ˜0.3%, in contrast to those of TPD-Si₂ (˜1.2%)and TAA (˜9%). The TPD-Si₂-functionalized device is most efficient,producing a maximum light output ˜10× greater than the Cu(Pc)-buffereddevice, and ˜100× greater than the bare ITO-based device. Improvementsobserved and differences between TPD-Si₂ and silyl-functionalized TAAmolecular layers can be explained, without limitation, in relation to:(1) closer aromatic structural similarity of TPD-Si₂ to TPD, producing astronger interlayer affinity and presumably greater π-π interfacialoverlap, and (2) the higher triarylamine: siloxane linker ratio inTPD-Si₂, consistent with more facile hole hopping via densertriarylamine packing.

Example 16

[0103] These findings argue that promotion of ITO-TPD interfacialcontact/adhesion leads to more efficient hole injection due to reducedinterfacial contact resistance. This hypothesis was tested by examiningcharacteristics of hole-only devices having the structure ITO/molecularinterlayer/TPD(250 nm)/Au(6 nm)/Al(80 nm), in which electron injectionat the cathode is blocked. Here the Au layer is deposited byrf-sputtering to avoid excessive heating of the TPD underlayer. The holeinjection capacity falls in the order TPD-Si₂≧TAA>Cu(Pc)>bare ITO (FIG.14). Compared to bare ITO, the silyl-functionalized TAA- andTPD-Si₂—modified anodes enhance the hole current density by 10-100× forthe same field strength, with ITO/TPD-Si₂ being most effective. Thus,when contact resistance at the OLED anode side is reduced and holeinjection increased, the greater electric field induced across the Alqlayer enhances electron injection and transport, affording higher lightoutput and comparable or, in the cases where recombination is moreprobable, enhanced quantum efficiency. In contrast, the present andrelated data for Cu(Pc) devices show that the Cu(Pc) significantlysuppresses hole injection. E. W. Forsythe, M. A. Abkowitz, Y. Gao, J.Phys. Chem. B 2000, 104, 3948; S. C. Kim, G. B. Lee, M. Choi, Y. Roh, C.N. Whang, K. Jeong, Appl. Phys. Lett. 2001, 78, 1445. It is believed, inlight of these results, that Cu(Pc) enhances quantum efficiency viabetter balancing hole and electron injection fluences, rather than byfacilitating hole injection or interfacial stability.

Example 17

[0104] To examine cohesion and crystallization effects on devicedurability, thermal stress tests were carried out on devices based onbare ITO, having a 10 nm Cu(Pc) interlayer, and having a 40 nm TPD-Si₂interlayer. These were subjected to heating at 95° C. for 0.5 h invacuum and subsequently examined for changes in luminous response. Theirreversible degradation of the bare ITO and Cu(Pc)-based devices uponheating at 95° C. for 0.5h (FIG. 15) is reasonably ascribed to TPDde-wetting and Cu(Pc)-nucleated TPD crystallization, respectively. Bothprocesses would disrupt the multilayer structure, leading to direct holeinjection into, and consequent degradation of, the emissive Alq layer,and possible amplification of pinholes and defects. In contrast,TPD-Si₂-buffered molecular layer devices exhibit enhanced performanceafter heating, which is presumably a consequence of interfacialreconstruction that promotes charge injection. These experimentsunambiguously demonstrate that covalently interlinkedalkylsilyl-substituted compounds such as TPD-Si₂ and TAA, when used asdescribed herein, offer significant improvements in stabilizing theanode-HTL interface and promoting hole injection.

[0105] The results of this and several preceding examples, demonstratethat a spincoated, hole injecting TPD-Si₂ layer can significantlyincrease maximum OLED device luminence (˜100×) and quantum efficiency(˜6×) by promoting ITO-TPD interfacial cohesion, hence promoting moreefficient hole injection. Devices having a TPD-Si₂ anode adhesion layerafford a maximum luminance level of 15,000 cd/m² in absence of dopantsor low work function cathodes, while exhibiting excellent thermalstability. In addition, the same results demonstrate that Cu(Pc)interlayers nucleate TPD crystallization upon heating above the T_(g) ofTPD

Example 18

[0106] The synthesis of alkylsilyl-functionalized TAA is as waspreviously described, both herein and in the literature. W. Li, Q. Wang,J. Cui, H. Chou, T. J. Marks, G. E. Jabbour, S. E. Shaheen, B. Kippelen,N. Pegyhambarian, P. Dutta, A. J. Richter, J. Anderson, P. Lee, N.Armstrong, Adv. Mater. 1999, 11, 730. TPD, Alq, and Cu(Pc) were obtainedfrom Aldrich and purified by gradient sublimation. All other reagentswere used as received unless otherwise indicated. TPD-Si₂ can besynthesized as provided elsewhere, herein (see Example 2) or accordingto FIG. 11B and Examples 19-21, and further characterized by ¹H NMRspectroscopy and elemental analysis.

Example 19

[0107] With reference to FIG. 11B enroute to TPD-Si₂, the synthesis of4,4′-bis[(p-bromophenyl)phenylamino)biphenyl (9). To a solution oftris(dibenzyldeneacetone)dipalladium (0.55 g, 0.6 mmol), andbis-(diphenylphosphino)ferrocene; (0.50 g, 0.9 mmol) in toluene (50 mL),was added 1,4-dibromobenzene (18.9 g, 0.0800 mol) at 25° C., and thesolution stirred under N₂ for 10 min. Subsequently, sodium tert-butoxide(4.8 g, 0.050 mol) and N,N′-diphenylbenzidine (6.8 g, 0.020 mol) wereadded, and the reaction mixture stirred at 90° C. for 12 h. The reactionmixture was subsequently cooled to 25° C. and poured into water. Theorganic layer was separated, and the aqueous layer was extracted withtoluene (3×100 mL). The extract was combined with the original organiclayer, and the solvent was removed in vacuo to give the crude product.This was purified by chromatography on silica gel using hexane: ethylenechloride (6:1) as the eluant. A white solid (6.9g) was obtained in 50%yield. ¹H NMR (CDCl₃): δ6.99(d, J=8.8 Hz, 4H), 7.02-7.16(m, 10H),7.28(t, J=7.6Hz, 4H), 7.34(d, J=8.8Hz, 4H), 7.45(d, J=8.4Hz, 4H).

Example 20

[0108] With further reference to FIG. 11B enroute to TPD-Si₂, thesynthesis of 4,4′-bis[(p-allylphenyl)′phenylamino]biphenyl (10). To astirring, anhydrous ether solution (10 mL) of 1(1.02 g, 1.58 mmol) underN₂ was added dropwise at 25° C. 1.6 mL (3.5 mmol) n-butyl lithium(2.5 Min hexanes), and the mixture stirred for 2 h. CuI (0.76 g, 4.0 mmol) wasthen added, the reaction mixture cooled to 0° C., and allyl bromide(0.60 g, 5.0 mmol) added in one portion. The solution was stirred for 14h, after which time it was quenched with 100 mL saturated aqueous NH₄⁺Cl⁻ solution, followed by extraction with ether (3×100 mL). Thecombined ether extracts were washed with water (2×100 mL) and brine(2×100 mL), and dried over anhydrous Na₂SO₄. Following filtration,solvent was removed in vacuo to yield a yellow oil. Chromatography onsilica gel with hexane: ethylene chloride (4:1) afforded 0.63 g whitesolid. Yield, 70%. ¹H NMR (CDCl₃) δ3.40(d, J=10 Hz, 4H), 5.10-5.20(m,4H), 6.02(m, 2H), 6.99-7.10(m, 2H), 7.10-7.20(m, 16H), 7.28(t, J=7.6Hz,4H), 7.46(d, J=8.8Hz, 4H). Anal. Calcd for C₄₂H₃₆N₂: C 88.68, H 6.39, N5.23 Found, C 87.50, H 6.35, N 4.93.

Example 21

[0109] With reference to FIG. 11B enroute to TPD-Si₂, the synthesis of4,4′-bis[(p-trichlorosilylpropylphenyl)phenylamino]biphenyl (11). To asolution of 2 (0.32 g, 0.55 mol) in 30 mL CH₂Cl₂ at 25° C. was added agrain of H₂PtCl₆.xH₂O, followed by HSiCl₃ (0.73 g, 5.5 mmol). Thereaction solution was warmed to 30° C. and stirred for 4 h. Removal ofthe solvent in vacuum yielded a dark-yellow oil. A mixture of 50 mLpentane and 10 mL toluene was then added. The resulting solid wasfiltered off, and the filtrate was concentrated under vacuum to aviscous, pale-yellow oil. Yield, 98%. ¹H NMR (CDCl₃): δ1.45(t, J=7Hz,4H), 1.90(t, J=7Hz, 4H), 2.70(brs, 4H), 6.80-7.80(m, 26H). Anal. Calcdfor C₄₂H₃₈Cl₆N₂Si₂: C 60.07, H 4.57; Found, C 60.52, H 4.87.

Example 22

[0110] With reference to Examples 19-21, a wide variety ofarylalkylamine molecular components and their silyl-functionalizedanalogs can be prepared using straight-forward modifications of thesynthetic techniques described herein. For instance, with reference toExample 19, diphenylbenzidene can be mono- or dialkylated with theappropriate haloalkyl reagent to provide the desired arylalkylamine holetransport layer component. As would also be well known to those skilledin the art made aware of this invention, the correspondingsilyl-functionalized molecular layer component can be prepared via mono-or dialkylation with the appropriate dihaloalkyl reagent followed bysubsequent silation, adopting the procedures illustrated in Examples 20and 21. Accordingly, by way of further example, the alkylated mono- anddiarylamine components, discussed above, and their silyl-functionalizedanalogs can be prepared to provide the structurally-related molecularand hole transport layers of this invention, and the enhancedperformance and/or hole injection resulting therefrom.

Example 23

[0111] TPD-Si₂ and TAA Thin Film Deposition and Characterization. Indiumtin oxide (ITO) glass sheets with a resistance of 20 Ω/□ from DonnellyCorp. were subjected to a standard literature cleaning procedure. TAAand TPD-Si₂-based buffer layers were spincoated onto cleaned ITOsurfaces from their respective toluene solutions (10 mg/mL) at 2 Krpm,followed by curing in moist air at 110° C. for 15 min. Cyclicvoltammetry of spincoated TPD-Si₂ films on ITO was performed with a BAS100 electrochemical workstation (scan rate, 100 mV/s; Ag wirepseudo-reference electrode, Pt wire counter electrode, supportingelectrolyte, 0.1 M TBAHFP in anhydrous MeCN). For TPD-Si₂ filmcontiguity assessment, 1.0 mM ferrocene in 0.1 M TBAHFP/MeCN was used asthe probe. Thermogravimetric analysis (TGA) was carried out on an SDT2960 DTA-TGA instrument with a scan rate of 10° C./min under N₂. TGAsample preparation involved drop-coating a TPD-Si₂ solution in toluene(10 mM) onto clean glass substrates under ambient conditions. Followingsolvent evaporation, the TPD-Si₂-coated slides were cured at 120° C. for1 h. Upon cooling, the films were detached from the glass substratesusing a razor blade and collected as powders for TGA characterization.

Example 24

[0112] ITO/Buffer Layer/TPD Interfacial Stability Studies. TPDde-cohesion analysis of the interfacial structures ITO/buffer layer/TPD(100 nm) (spincoated TPD-Si₂, spincoated TAA, vapor-deposited Cu(Pc))were carried out in the following manner. Following vapor deposition of50-100 nm TPD films onto the respective buffer layer-coated ITOsubstrates, the samples were annealed at 80-100° C. under N₂ for 1.0 h,and the film morphology subsequently imaged by optical microscopy andAFM.

Example 25

[0113] OLED Device Fabrication. OLED devices of the structure:ITO/interlayer/TPD(50 nm)/Alq(60 nm)/Al(100 nm) were fabricated usingstandard vacuum deposition procedures (twin evaporators interfaced to a<1 ppm O₂ glove box facility). Deposition rates for organic and metalwere 2-4 Å/sec and 1-2 Å/sec respectively, at 1×10⁻⁶ Torr. The OLEDdevices were characterized inside a sealed aluminum sample containerunder N₂ using instrumentation described elsewhere. J. Cui, Q. Huang, Q.Wang, T. J. Marks, Langmuir 2001, 17, 2051; W. Li, Q. Wang, J. Cui, H.Chou, T. J. Marks, G. E. Jabbour, S. E. Shaheen, B. Kippelen, N.Pegyhambarian, P. Dutta, A. J. Richter, J. Anderson, P. Lee, N.Armstrong, Adv. Mater. 1999, 11, 730.

Example 26

[0114] Device Thermal Stability Evaluations. OLED devices were subjectedto heating under vacuum at 95° C. for 0.5 h, and were subsequentlyevaluated for I-V and L-V characteristics as described above.

[0115] While the principles of this invention have been described inconnection with specific embodiments, it should be understood clearlythat these descriptions are added only by way of example and are notintended to limit, in any way, the scope of this invention. Forinstance, the present invention can be applied more specifically to theconstruction of second-order nonlinear optical materials as have beendescribed in U.S. Pat. No. 5,156,918 which is incorporated herein byreference in its entirety. Likewise, the present invention can be usedin conjunction with the preparation of optical waveguides. Anotheradvantages and features will become apparent from the claimshereinafter, with the scope of the claims determined by the reasonableequivalents, as understood by those skilled in the art.

1. A method of using an amine molecular component to enhance holeinjection across the electrode-organic interface of a light emittingdiode device, said method comprising: providing an anode; andincorporating an electroluminescent medium adjacent said anode, saidmedium comprising an amine molecular layer, coupled to said anode, saidmolecular layer having at the least one of an arylamine molecularcomponent and an arylalkylamine molecular component, each said componentsubstituted with at least one silyl group, and on said molecular layer ahole transport layer of molecular components having said aminestructure.
 2. The method of claim 1 wherein said molecular layercomponents are selected from the group consisting ofalkylsilyl-substituted compounds of FIGS. 2A and 2C.
 3. The method ofclaim 2 wherein said molecular layer components are selected from thegroup consisting of alkylsilyl-substituted TAA andalkylsilyl-substituted TPD.
 4. The method of claim 3 wherein saidmolecular layer is spin-coated on said anode.
 5. The method of claim 3wherein said anode is immersed in a solution of said molecular layercomponents.
 6. The method of claim 1 wherein a plurality of molecularlayers are coupled to said anode.
 7. The method of claim 1 wherein saidhole transport layer is TPD.
 8. The method of claim 7 wherein said holetransport layer is spin-coated on said anode.
 9. An electroluminescentdevice for generating light upon application of an electrical potentialacross two electrodes, said device comprising: an anode; at least oneamine molecular layer, coupled to said anode, said molecular layerhaving at least one of an arylamine molecular component and anarylalkylamine molecular component, each said component substituted withat least one silyl group; a conductive layer of molecular componentshaving said amine structure; and a cathode in electrical contact withsaid anode layer.
 10. The device of claim 9 wherein said molecular layercomponents are selected from the group consisting ofalkylsilyl-substituted compounds of FIGS. 2A and 2C.
 11. The device ofclaim 10 wherein said molecular layer components are selected from thegroup consisting of alkylsilyl-substituted TAA andalkylsilyl-substituted TPD.
 12. The device of claim 11 wherein saidconductive layer is a hole transport layer of TPD.
 13. The device ofclaim 9 wherein a plurality of molecular layers are coupled to saidanode.
 14. An electroluminescent device for generating light uponapplication of an electrical potential across two electrodes, saiddevice comprising: an anode; at least one molecular layer, coupled tosaid anode, of arylamine molecular components substituted with at leasttwo silyl groups; a hole transport layer of TPD molecular components,said hole transport layer substantially without crystallization uponannealing; and a cathode in electrical contact with said anode.
 15. Thedevice of claim 14 wherein said molecular layer components are selectedfrom the group consisting of alkylsilyl-substituted TAA andalkylsilyl-substituted TPD.
 16. The device of claim 14 wherein aplurality of molecular layers are coupled to said anode.
 17. The deviceof claim 14 further including an electron transport layer.
 18. Anelectroluminescent device for generating light upon application of anelectrical potential across two electrodes, said device comprising; ananode; at least one molecular layer, coupled to said anode, ofalkylsilyl-substituted TPD molecular components; a hole transport layerof TPD molecular components and a cathode in electrical contact withsaid anode.
 19. The device of claim 18 further including, an electrontransport layer.
 20. The device of claim 18 wherein a plurality ofmolecular layers are coupled to said anode.